FeS Colloids Trigger Antimony Redox Cycling, Colloid Formation, and Ultimate Fate during the Anoxic–Oxic Transition

Sb(III) was measured directly with Sb(V) masked by 10 % citric acid. Sb(V) was calculated as the difference between total Sb and Sb(III). Fe(II) was determined photometrically at 510 nm using a modified 1,10‑phenanthroline method.

Sulfide was analyzed by the methylene blue method at 665 nm. Thiosulfate and sulfate were measured by ion chromatography (ICS‑90). Elemental sulfur (S(0)) in unfiltered solids was methanol-extracted and quantified by HPLC. •OH was trapped with 20 mM BA, and the resulting p-hydroxybenzoic acid (p‑HBA) was quantified by HPLC and converted to •OH concentration using a yield factor of 5.87.

Labile Fe(III) was extracted and quantified using established methods based on xylenol orange disodium salt (XO) as a specific chelating agent. Additional procedural details are provided in Section S5.Size Fractionation. Sb and Fe species were quantified following separation of the suspension into three size fractions: truly dissolved (< 10000 Da, approximately < 1–3 nm), colloidal (10000 Da–450 nm), and particulate (> 450 nm).

Fractionation was performed by 10 kDa ultrafiltration (Amicon Ultra‑15) and 0.45 μm PES filtration (Whatman), as detailed in Section S6. Colloids are operationally defined as particles ranging from 10 kDa to 0.45 μm.Density Functional Theory (DFT) Calculations. Gaussian-based theoretical calculations were performed using an Fe4S4 cluster as a representative model of FeS, along with the Sb(OH)3 and Sb(OH)6– species, to gain molecular-level insight into FeS-mediated Sb redox cycling during anoxic-oxic transitions.

Full computational details are given in Section S7.Characterization of Colloids and Secondary Mineral Precipitates. The particle size distribution and zeta potential of the suspensions collected at predetermined intervals were determined by DLS. Aggregation kinetics of colloids collected after 30 min of oxidation were monitored using time-resolved DLS.

The morphology and microscopic structure of colloids and/or secondary minerals collected at 30 min and 240 min of oxidation were examined using an ultrahigh-resolution aberration-corrected scanning transmission electron microscopy (Cs-STEM, Thermo Scientific). Samples were prepared by pipetting 30 μL of the suspension onto a lacey carbon film supported by a 200‑mesh copper grid (01895‑F, Ted Pella, USA), which was subsequently dried inside an anoxic glovebox to avoid oxidation.The suspension samples collected at selected reaction time points were centrifuged and freeze-dried to obtain solids for X-ray absorption spectroscopy (XAS), Mössbauer spectroscopy, cryogenic X-ray photoelectron spectroscopy (cryo‑XPS), X-ray diffraction (XRD), Raman, and Fourier transform infrared (FTIR) spectroscopy characterization.

XRD patterns were collected using Cu Kα radiation (XRD‑6100, Shimadzu). Raman spectra were acquired with a Renishaw spectrometer, and FTIR spectra were recorded using a Bruker Vertex 70 spectrophotometer. Cryo‑XPS spectra were collected at −160 °C using a PHI 5000 VersaProbe III SXM scanning X-ray microprobe equipped with a monochromatic Al Kα source.

Mössbauer spectra were recorded at 6.2 K in transmission geometry using a Wissel SLD‑500 spectrometer (Germany) at the Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Sb K‑edge X‑ray absorption near‑edge structure (XANES) and extended X‑ray absorption fine structure (EXAFS) spectra were acquired at room temperature on beamline BL14W1 at the Shanghai Synchrotron Radiation Facility (SSRF). Detailed descriptions of these characterization procedures are provided in Section S8.Extraction Experiments.

After the anoxic‑oxic reactions, surface-bound Sb was extracted by subjected the suspension samples with 1 M NaH2PO4 for 16 h. The extractable Sb fraction was determined from the extract, and the non‑extractable fraction was calculated as the difference between total Sb and extractable Sb. Detailed procedures are provided in Section S9.